Present day toners are formulated from a range of potential components. Most toner compositions include at least a polymeric binder material and a colorant. Other commonly used components include black and colored magnetic oxides, charge control agents, internal additives to augment toner properties, such as aiding in deagglomeration and homogeneous distribution of the colorant in the toner composition, and external additives, to aid in the proper function of the toner. The components used in a particular toner formulation are dependent on the requirements of the machine in which the toner will be used. For instance, the toner formulation must take into account such parameters as image quality, reliability, carrier life, toner shelf life, etc., all of which are intricately involved with the mechanical capability and design of the hardware of the machine. Often, there is more than one component of a toner formulation which performs to eradicate certain undesirable properties of the toner. These same components may however, also contribute to other problems, or the combination of two or more components which affect the same toner properties may result in over-correction of a trouble area in the toner performance. Therefore, the combination of components selected to comprise a given toner composition must be carefully balanced, taking into account the full range of properties contributed by each component and the interaction of each component with every other component of the toner composition. The choice of components is further influenced by cost and other practical considerations.
The bulk polymeric material of the toner generally functions as the binder for the colorants included in the toner formulation, but also affects many of the other toner functions, such as charging, electrical resistivity, and mechanical integrity, to name a few. Therefore, often times a combination of resins is used to achieve the desired performance. Polymers used in toner may be linear, branched or cross linked, and are chosen for their properties with respect to: thermal performance, i.e., glass transition temperature, melt viscosity, blocking temperature, and thermal integrity; mechanical properties, i.e., impact strength, adhesive/cohesive strength, and surface energy; electrical traits, i.e., triboelectric charge function, resistivity, and dielectric constant; and other miscellaneous features, such as moisture resistivity, % volatility, molecular weight, colorlessness, and pigment compatibility.
Among the most popular resins from which the toner resin may be selected are: acrylic resins, epoxy resins, polyamide resins, polyester resins, polyethylene resins, polystyrene resins, styrene-acrylic copolymer resins, and styrene-butadiene resins.
Dispersed in the binder resin are the colorants used in the toner formulation. In monocomponent toners, magnetic oxide pigments are used for the purpose of enhancing the magnetic attraction between the toner and the developer roll assembly. Carbon black has historically been the most popular colorant used in black toners, as it strongly influences the triboelectric charging capability of the toner. However, more recent toners employ charge control agents to achieve and control this toner feature, thus allowing the use of more easily dispersed black colorants. The black colorant may also affect the flow characteristics of the toner and, therefore, is sometimes added in incremental amounts to the toner surface.
The use of charge control agents to affect charging and development has been known. Originally, the agents of choice were those comprising metal dye complexes. These materials, however, are high in cost, color, and often times may exhibit unacceptable toxicity levels. Charge reversal agents, most of which are negatively charged, became popular with use of the organic photoreceptor. The negative charge imparted generated a need to balance the overall charge on the toner/system. This need was addressed by the development of coated carrier materials, such as fluorocarbon-coated ferrite carriers. Also, and in combination with the coated carrier, it has been popular to use nitrogen containing nigrosine dyes. These dyes, however, not unlike the metal dye complexes, are highly colored and contribute to a host of other problems relating to uniformity and reproducibility in print, carrier contamination, and quality consistency. No one charge control agent is known which is suited for all machines, and in choosing an agent the technician must consider machine hardware, the carrier component characteristics, the polymer material characteristics, the colorants, and processing conditions. As was noted earlier, it is necessary to balance the properties of the various toner components when addressing these considerations.
Most toner formulations also include any one or more of a number of materials known commonly in the industry as additives. These are generally fine particles which are physically blended with the toner at up to about 3% of the composition. They may be attached to the toner by electrical means, mechanical means, or by mere physical mixing, though this is not generally the manner of choice. These additives may be added to influence flow control, charge control, cleaning, fixing, offset prevention, transfer, conductivity control, humidity sensitivity control, and carrier life stability. Common additive materials include silica, metal oxides, metal stearates, fluoro polymer powders, fine polymer powders, rare earth oxides, waxes, conductive particulates, magnetite, carbon, and titanates. Choice of additives is critical, however, given that many of the additives affect more than a single property.
Clearly, given the vast number of components available in the industry for use in toner compositions, and given the propensity for many of the components to enhance some properties and at the same time to deleteriously affect others, choice of components is clearly not a routine matter.
For example, it is known, as was set forth earlier, to produce toner compositions which include charge control agents, and even to produce toner compositions which include a combination of charge control agents. Further it is known to use agents which are metal complexes of certain acids. These compositions do not always, however, impart to the toner composition the appropriate or necessary charge level. It has remained for the current inventors to develop a toner composition including a charge control agent combination which desirably affects the resulting toner as it relates to performance in the machine in which it is intended to be used.
Also of concern, has been the choice of post additives used in toner formulations. The current inventors have also developed a combination of post additives, to be used as a surface treatment for toner compositions, which results in desirable toner performance.
The combination of these discoveries, though each alone is found to result in unexpected performance advantages, results in even greater benefits in use than the components impart individually.